This invention relates to a circuit which is connected in a circuit extended from a power amplifier to loudspeakers thereof for balancing the sound volumes of the right and left loudspeakers or of the front and rear loudspeakers. This is, this invention relates to a variable resistor employed in a fader control circuit.
The fader control circuit is extensively employed for car radio receiver sets. For instance, in the case where a radio receiver set is provided near the operator's seat of a car, the fader control circuit is used so that the sound volumes of the right and left loudspeakers or of the front and rear loudspeakers can be balanced by a person on the rear seat. Alternatively, in the case where the loudspeakers are positioned at the right, left, front and rear sides of a person, the fader control circuit is used to adjust (balance) the sound volumes of the front and rear loudspeakers by adjusting the outputs of the right and left power amplifiers. In other words, the circuit is used to control the output of a power amplifier with a variable resistor.
Examples of the fader control circuit are as shown in FIGS. 1 and 2, which comprises: a power amplifier A, which can be either the right and left, or front and rear, loudspeakers SP.sub.1 and SP.sub.2 ; and a variable resistor VR for adjusting output levels applied to the loudspeakers SP.sub.1 and SP.sub.2. As the sliding piece (wiper) of the variable resistor VR is moved, the power applied to the loudspeakers SP.sub.1 and SP.sub.2 is changed; that is, the sound volumes of the loudspeakers are changed.
In general, the variable resistor VR is 40 to 80.OMEGA. in maximum resistance and 10 watts in capacity. The output power of the power amplifier A is, by comparison high, 20 to 40 watts in maximum in the car of automobile hifi equipment. Accordingly, when the input and output terminals of the variable resistor VR are shorted or opened, i.e. when for instance the voice coil of the loudspeaker SP.sub.1 is shorted or the voice coil of the loudspeaker SP.sub.2 is broken, and the sliding piece of the variable resistor is positioned as indicated by the dotted line in the figure, then all of the output current of the power amplifier A is caused to flow through a small part R of the resistance member of the variable resistor VR. As a result, heat is abnormally generated in the small part R while increasing the temperature thereof to, for instance, several hundreds of degrees. This temperature increase may cause a fire.
In order to eliminate this difficulty, or to prevent such a fire, heretofore a method is employed in which a protection circuit is provided in a relevant device to detect the short-circuiting of one or both output terminals of the variable resistor. When this occurs the device is deenergized when the short-circuiting occurs. However, the method is disadvantageous in that the protection circuit is expensive and the operation thereof is relatively unreliable.